s390/vdso: put vdso datapage in a separate vma

Add a separate "[vvar]" mapping for the vdso datapage, since it
doesn't need to be executable or COW-able.

This is actually the s390 implementation of commit 87154938
("arm64: vdso: put vdso datapage in a separate vma")

Reviewed-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>